1. Field of the Invention
The present invention relates to a signs-of-deterioration detector for a semiconductor laser that detects signs of deterioration of a semiconductor laser before the semiconductor laser no longer meets the operation requirements thereof.
2. Description of the Related Art
Semiconductor laser devices have been employed in recent years in various technology sectors including those of telecommunications, recordings, sensors and electrophotographic exposures. The scope of applications of semiconductor laser devices is expected to expand in future particularly in the medical sector.
In the above-identified circumstances, there is an increasing demand for devices having a long and stable service life.
Besides, there is also a demand for systems that allow an operationally failed device to be replaced quickly and also for equipment that operates without any loss of time. Then, consequently, there is a demand for apparatus that can detect conditions of a device that can lead to an operation failure before the device actually fails to operate.
Thus, there arises a need of predicting a situation where a device can no longer meet the operation requirements thereof considerably before it actually gets into such a situation.
Japanese Patent Application Laid-Open No. 2006-278662 proposes a light source for optical communications wherein the optical output of the light source is monitored and a countermeasure is taken whenever the optical output falls below a reference value as a technique of detecting a change in a device that makes the device no longer meet the operation requirements thereof.
FIG. 9 schematically illustrates an arrangement disclosed in Japanese Patent Application Laid-Open No. 2006-278662. Referring to FIG. 9 illustrating a laser module 18 that operates as light source for optical communications, the output light from each of the surface-emitting lasers arranged on a substrate 13 in an integrated manner are transmitted through a transparent and flexible substrate 15 and made to enter the core 17 of an optical fiber 16.
The output light from each of the surface-emitting lasers (the output light leaking to the side opposite to the emission side) is constantly monitored by a corresponding photodiode 301 and, when the fall of the output of any one surface emitting laser exceeds a predetermined threshold value as determined by a control circuit 19, a switching circuit 20 is actuated so as to drive another one of the surface-emitting lasers.
However, the above-described arrangement relying only on a change in the optical output of a surface emitting laser entails a risk that no deterioration of the surface emitting laser is detected or that the surface emitting laser is determined to be deteriorated while the laser is actually not deteriorated at all, when the optical output is sensitive to the temperature of the surrounding environment and changes accordingly.
Additionally, providing a switching element in the vicinity of such a laser module in all applications may be difficult from the viewpoint of space and cost of device arrangement.
The above-identified problems will be described more specifically by referring to some of the accompanying drawings.
FIG. 4 is a graph schematically illustrating the initial characteristics of the electric current dependency of the optical output of a semiconductor laser device relative to the temperature of the surrounding environment.
According to FIG. 4, with the initial characteristics that are observed before any device deterioration, the optical output at current value A falls by about 10% when the temperature of the surrounding environment changes from 50° C. to 60° C., for example.
Therefore, if there is an arrangement that the optical output value of a device is monitored and a countermeasure, which may be replacing the device for example, is taken when the optical output value exceeds a certain reference value, the net result may be that the device is replaced although the device has not been deteriorated and is still operable to a great disadvantage from the viewpoint of cost and downsizing. On the other hand, even if the device has been deteriorated and the optical output of the device is on the way of rapidly decreasing, the optical output may increase by about 10% with the current value A when the temperature of the surrounding environment changes from 50° C. to 40° C. as seen from FIG. 4.
Note that such an actual increase of the optical output represents the temperature characteristics in a deteriorated state of the device and, if the temperature characteristics in the deteriorated state are different from the initial characteristics of the device, the optical output of the device cannot be accurately figured out from FIG. 4. In other words, the figured out optical output may only be useful as reference value.
Thus, deterioration of the device cannot be detected if the temperature characteristics of the device that is on the way of deterioration ‘absorb’ the deterioration. Then, replacement of the device will take place after due time and there will be a time period where no device is operable, which represents a pure loss of time.
FIG. 3 illustrates the change with time in the optical output of a semiconductor laser device observed under a condition where the device is driven with a constant electric current value and the temperature of the surrounding environment is held to a constant level.
More specifically, in FIG. 3, solid line 101 illustrates how the optical output of a semiconductor laser device changes with time under a condition where the device is driven with a constant electric current value.
According to FIG. 3, the optical output of the device is held stable for a long period of time after the device is driven to start operating but declines gradually when a certain period of time has passed. Thereafter, as time goes by further, the optical output falls dramatically.
Referring to FIG. 3, the period of time during which the optical output falls dramatically that is indicated by a dotted chain line 102 will be referred to as “optical-output-rapidly-decreasing period” in this specification.
This period is a period where the device is being rapidly deteriorated.
If the optical output of the device is required to be above value 104, which is specified as an operation requirement of the device, the device no longer meets the operation requirement after time point T2′.
If an optical output of value 103, which is higher than value 104, is defined as deterioration threshold value, deterioration of the device is detected with the known method at time point T2 when the optical output of the device falls below 103.
Since time point T2 is already in the optical-output-rapidly-decreasing period, the time span given to cope with the deterioration of the device is very short. More specifically, if the time span spent to cope with the deterioration of the device after the detection of deterioration exceeds (T2′-T2), there arises a time period when the device is no longer operable.
On the other hand, if the reference value 103 to be used for the purpose of detection of deterioration of the device is made closer to the initial stable optical output level in order to detect the deterioration of the device earlier, the difference in the optical output to be detected becomes smaller than the fluctuations in the optical output that may be caused by factors other than deterioration such as temperature changes as pointed out earlier.
Then, the operation of detecting a fall in the optical output due to deterioration early and catching signs of deterioration will become a very difficult one.
Thus, with a method that relies only on a change in the optical output, the system incorporating an semiconductor laser device is not operable and hence is not operating at all when the deteriorated device is being replaced to consequently give rise to a loss of time because the time when the deterioration of the device is detected is found in the optical-output-rapidly-decreasing period.